The present invention relates to printing of patterns with extremely high precision on photosensitive surfaces, such as photomasks for semiconductor devices and displays. More specifically the invention relates to a system for creating a pattern on a workpiece comprising a source for emitting light pulses in the wavelength range from EUV to IR, a spatial light modulator (SLM) having at least one modulating element (pixel), adapted to being illuminated by at least one emitted light pulse and a projection system creating an image of the modulator on the workpiece.
It is previously known, e.g. from WO 99/45439 by the same applicant, to use a spatial light modulator (SLM) in a pattern generator. This has a number of advantages compared to the more wide-spread method of using scanning laser spots: the SLM is a massively parallel device and the number of pixels that can be written per second is extremely high. The optical system is also simpler in the sense that the illumination of the SLM is non-critical, while in a laser scanner the entire beam path has to be built with high precision. Compared to some types of scanners, in particular electro-optic and acousto-optic ones, the micromirror SLM can be used at shorter wavelengths since it is a purely reflective device. Such a pattern generator comprises a source for emitting light pulses, an SLM with modulating elements (pixels), adapted to being illuminated by the emitted light pulses and a projection system creating an image of the modulator on the workpiece.
However, a problem with using SLM in pattern generators is that for practical reasons each feature on the workpiece has to be produced by one or at least very few light pulses. Consequently, the system becomes very sensitive to flash-to-flash energy variations and time jitter. These problems are especially important in gas discharge lasers, such as in a normally used excimer laser. A conventional excimer laser has flash-to-flash energy variations of 5% and flash-to-flash time jitter of 100 ns. These variations are due to various factors such as variations in the gain medium and variations in the electrical discharge process. The duration of the laser pulse for a typical excimer laser used for lithography is about 10-20 ns and the pulse frequency is in the range of about 1,000 Hz. By using two exposures for each feature on the workpiece this problem could to some extent be alleviated, but not totally eliminated.
In conventional microlithography, i.e. wafer steppers or wafer scanners, using light flashes, such as in integrated circuit lithography, less precision of the light pulses is required, since each feature on the workpiece could normally be created by 50 light pulses or more. Consequently, the integrated exposure on each part of the workpiece area becomes less sensitive to light pulse variations. However, even in this case flash-to-flash variations are troublesome. To this end it is proposed in U.S. Pat. No. 5,852,621 to control the laser pulse energy by using a fast pulse energy detector having response time in the nanosecond or sub-nanosecond range providing an electrical signal representing pulse energy to a trigger circuit. The trigger circuit integrates the signal and triggers an electro-optic switch, such as a Pockels cell, when the integrated signal reaches a predetermined level. The operation of the electro-optic switch trims a portion of the pulse energy so that the resulting pulse energy is maintained at a consistent level.
It is therefore an object of the present invention to provide an improved SLM pattern generator for printing of precision patterns.
This object is achieved with a system according to the appended claims.
It should be noted that the invention specifically relates to printing of photomasks for semiconductor devices and displays, but also relates to direct writing of semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. Furthermore, it can have applications to other types of precision printing such as security printing. The term printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is not limited to mean visible light, but a wide range of wavelengths from infrared (IR) to extreme UV.